Luminescent layers of material have commonly been used in various applications including fluorescent lighting tubes, cathode ray tubes, x-ray radiographic machines, and solid state electroluminescent panels. In each of these applications excess electrons or holes are produced by a source which is external to the luminescent layer. Photons of electromagnetic radiation, e.g., visible light, are than produced within the layer when electrons and holes recombine within the film. However, electrons and holes may also recombine at various defects within the layer without producing photons, resulting in a decrease of the luminescent efficiency.
In the fluorescent lighting tube, electrons and holes are excited within the luminescent layer by ultraviolet radiation generated within the tube. In the cathode ray tube, the luminescent layer is excited by a high energy incident electron beam. In the case of the x-ray radiographic apparatus, the luminescent layer is excited by a beam of incident x-rays. In the case of solid state electroluminescent devices, electrons or holes are produced within the luminescent layer by carrier injection from an electrode. In each case the luminescence efficiency depends upon the relative importance of recombination by emission of a photon and recombination at a crystalline defect without the emission of a photon.
Recombination without the emission of photons, e.g., light, or non-radiative recombination, occurs primarily on crystallographic defects in each of the crystallites of the luminescent layer. These crystalline defects, including grain boundaries, stacking faults, crystalline twins, dislocations, lattice vacancies, and deleterious impurity ions, are largely concentrated around the boundaries of each of the grains of the crystallite comprising the luminescent layer. In order to obtain a high efficiency from such a luminescent layer it is necessary that the defect containing material around the outside of each grain of the luminescent layer comprise a small portion of the total volume of the respective grain. This is achieved in the prior art by using grains or crystallites which are large relative to the electron diffusion length in the fabrication of the luminescent layer. Thus, minority carriers within a given grain have a higher probability for luminescent recombination within the grain then for non-radiative recombination in the defective region near the boundary of the grain.
In order to obtain a high efficiency of luminescence the grain comprising a luminescent layer of the various applications of the prior art are large relative to the charge carrier diffusion lengths therein. In a luminescent layer of the prior art, comprising materials such as zinc oxide, cadmium sulfide, or zinc sulfide, typical grain sizes range from about 100 microns to over 1,000 microns, in order to obtain high efficiencies.
In typical application, grains of these luminescent materials are applied to the surface an insulating substrate, such as glass, by means of a liquid vehicle. The liquid is allowed to evaporate after the suspension of luminescent grains within the liquid is applied to the supporting substrate. The individual grains of luminescent material are fused together and activated by a process of calcining, in which the layer of material is heated to a temperature of about 400.degree. to 600.degree. centigrade for several minutes in an atmosphere which may contain a chemically reducing gas mixture. The resulting luminescent layer is a loose aggregate of individual grains of luminescent material, each of which is from about 100 microns to over 1,000 microns across. Because of the large grain size the luminescent layer is translucent in appearance, and reflects light by the diffused scattering of incident photons. Additionally, in order to increase the efficiency of luminescent layers comprising zinc oxide, cadmium sulfide, or zinc sulfide, impurity dopants such as copper, manganese, iron and other metals are added to these materials in order to introduce active sites at which radiative recombination occurs efficiently. However, even with the addition of these luminescence enhancing dopants, it is necessary to use relatively large crystallites of material in order to achieve a high luminescent efficiency.
Several different advantages are inherent in the nature of luminescent layers of the prior art. It has been difficult to achieve a resolution which is better than several times the size of the larger grains comprising the luminescent layer of the prior art. One reason for this low resolution is that electrons or holes which are excited within one grain may produce light by recombination at any other point of that grain. Another reason for the low resolution of the prior art is the fact that light produced within the luminescent layer maybe scattered several times within the layer because of the loose agglomeration of the adjacent large grains comprising the layer. Taken together, the deficiencies of the luminescent layers of the prior art limit the resolution of high efficiency cathode ray tubes, x-ray radiographic apparatus, nuclear particle detectors, and various electroluminescent display schemes.
Another limitation of the luminescent layers of the prior art is the relatively high electrical resistance thereof. This is a result of the physical structure comprising the agglomeration of grains within the layer, and the large band gap materials comprising each of the grains.
Another limitation of the luminescent layers of the prior art is the very low relative efficiency of small grain polycrystalline thin films of luminescent materials because of the recombination of excess electrons or holes excited within the thin film at the grain boundaries contained therein. The prior art does not teach a technology for effectively eliminating the deleterious effect of minority carrier recombination at grain boundaries or surfaces.